1. Field of the Invention
The present invention relates to a refrigerating oil composition, and more particularly to a refrigerating oil composition which exhibits excellent lubrication properties when used in combination with certain types of coolant; i.e., a hydrofluorocarbon-type, fluorocarbon-type, hydrocarbon-type, ether-type, carbon dioxide-type, or ammonia-type coolant, preferably in combination with a hydrofluorocarbon-type coolant, which may serve as a substitute for chlorofluorocarbon coolants which have been implicated as causing environmental problems. The refrigerating oil composition of the present invention exhibits notably improved lubrication between aluminum material and steel material to thereby suppresses wear of the materials, and hardly causes clogging of capillary tubes.
2. Background Art
A compression-type refrigerator typically includes a compressor, a condenser, an expansion mechanism (such as an expansion valve), an evaporator, and in some cases a drier. A liquid mixture of a coolant and a refrigerating oil circulates within the closed system of the refrigerator. Conventionally, as coolant in compression-type refrigerators, particularly in air conditioners, there has widely been used chlorodifluoromethane (hereinafter referred to as R22) or a mixture of chlorodifluoromethane and chloropentafluoroethane at a weight ratio of 48.8:51.2 (hereinafter referred to as R502). As lubricating oils in such apparatuses, there have been employed a variety of mineral oils and synthetic oils that satisfy the aforementioned requirements. However, R22 and R502 have recently become more strictly regulated worldwide for fear of causing environmental problems, such as destruction of the ozone layer in the stratosphere. Therefore, as new coolants, hydrofluorocarbons typified by 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, and 1,1,1-trifluoroethane (hereinafter referred to as R134a, R32, R125, and R143a, respectively) have become of interest. Hydrofluorocarbons, inter alia, R134a, R32, R125, and R143a, involve no fear of destroying the ozone layer, and thus are preferable coolants for use with compression-type refrigerators. However, when used alone, hydrofluorocarbons have the following disadvantages (1)-(3), as reported in "Energy and Resources" Vol. 16, No. 5, page 474: (1) when R134a is used in an air conditioner in place of R22, operation pressure is low, resulting in an approximate 40% reduction in cooling performance and approximate 5% reduction in efficiency, as compared to the case of R22. (2) R32, though providing better efficiency than R22, requires high operation pressure and is slightly inflammable. (3) R125 is non-inflammable, but has low critical pressure and yields lowered efficiency. R143a, like R32, has the problem of inflammability.
Coolants for compression-type refrigerators are preferably used in existing refrigerators without necessitating any modification to them. In practice, however, due to the aforementioned problems, coolants should be mixtures which contain the above-described hydrofluorocarbons. That is, in creation of a substitute for currently employed R22 or R502, it is desirable to use inflammable R32 or R143a from the point of efficiency, and in order to make the overall coolant non-inflammable, R125 and R134a are preferably added thereto. "The International Symposium on R22 & R502 Alternative refrigerants," 1994, page 166, describes that R32/R134a mixtures are inflammable when the R32 content is 56% or higher. Coolants containing non-inflammable hydrofluorocarbons such as R125 or R134a in amounts of 45% or more are generally preferred, although this range is not necessarily an absolute one and may differ depending on the composition of the coolant.
In a refrigeration system, coolants are used under a variety of different conditions. Therefore, the composition of a hydrofluorocarbon to be incorporated into the coolant preferably does not change greatly from point to point within the refrigeration system. Since a coolant is present in two states--a gas state and a liquid state--in a refrigeration system, when the boiling points of hydrocarbons to be incorporated greatly differ, the composition of the coolant in the form of a mixture may greatly differ from point to point within the refrigeration system, due to the aforementioned reasons.
The boiling points of R32, R143a, R125, and R134a are -51.7.degree. C., -47.4.degree. C., -48.5.degree. C., and -26.3.degree. C., respectively. When R134a is incorporated into a hydrofluorocarbon-containing coolant system, its boiling point must be taken into consideration. When R125 is incorporated into a coolant mixture, its content is preferably from 20-80 wt. %, particularly preferably 40-70 wt. %. When the R125 content is less than 20 wt. %, coolants such as R134a having a boiling point greatly different from that of R125 must be added disadvantageously in great amounts, whereas when the R125 content is in excess of 80 wt. %, the efficiency disadvantageously decreases.
In consideration of the foregoing, preferable substitutes for conventional R22 coolants include mixtures containing R32, R125, and R134a in proportions by weight of 23:25:52 (hereinafter referred to as R407C) or 25:15:60; and mixtures containing R32 and R125 in proportions by weight of 50:50 (hereinafter referred to as R410A) or 45:55 (hereinafter referred to as R410B). Preferable substitute coolants for R502 coolants include mixtures containing R125, R143a, and R134a in proportions by weight of 44:52:4 (hereinafter referred to as R404A); and mixtures containing R125 and R143a in proportions by weight of 50:50 (hereinafter referred to as R507).
These hydrofluorocarbon-type coolants have different properties from conventional coolants. It is known that refrigerating oils which are advantageously used in combination with hydrofluorocarbon-type coolants are those containing as base oils certain types of polyalkylene glycol, polyester, polycarbonate, polyvinyl ether, or similar materials having specific structures, as well as a variety of additives such as antioxidants, extreme pressure agents, defoamers, hydrolysis suppressers, etc.
However, these refrigerating oils have poor lubrication properties in the aforementioned coolant atmosphere, and there arises notable increases in friction between aluminum material and steel material of refrigerators contained in air conditioners for automobiles, electric refrigerators, and household air conditioners, raising great problems in practice. The aluminum-steel frictional portions are important elements in compressors, and are found, for example, between a piston and a piston shoe, and between a swash plate and a shoe section in reciprocation-type compressors (particularly in swash plate-type compressors); between a vane and its housing in rotary compressors; and in the sections of an Oldham's ring and a revolving scroll receiving portion in scroll-type compressors.
A refrigerator is equipped with an expansion valve called a capillary tube. The capillary tube is a thin tube having a diameter of as small as 0.7 mm and thus is apt to become plugged. The plugging phenomenon of a capillary tube is a critical factor that determines the service life of the refrigerator.
Therefore, in the case in which hydrofluorocarbon coolants are used as substitutes for chlorofluorocarbon coolants, there has been need for refrigerating oils which are endowed with excellent lubrication properties, inter alia, improved lubrication between aluminum material and steel material, which suppress friction, and which hardly cause plugging of a capillary tube.